Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2020	Jul. 24, 2020
Cover [Abstract]
Entity Registrant Name	Alkermes plc.
Entity Central Index Key	0001520262
Document Type	10-Q
Document Period End Date	Jun. 30, 2020
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		159,043,691
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q2
Trading Symbol	ALKS
Entity File Number	001-35299
Entity Incorporation, State or Country Code	L2
Entity Tax Identification Number	98-1007018
Entity Address, Address Line One	Connaught House
Entity Address, Address Line Two	1 Burlington Road
Entity Address, City or Town	Dublin 4
Entity Address, Country	IE
Entity Address, Postal Zip Code	D04 C5Y6
City Area Code	353
Local Phone Number	1-772-8000
Document Quarterly Report	true
Document Transition Report	false
Title of 12(b) Security	Ordinary shares, $0.01 par value
Security Exchange Name	NASDAQ